The present invention relates to an electrode of the non-sintered type including a three-dimensional current collector and suitable for use in a secondary electrochemical cell having an alkaline electrolyte, in particular a cell of the nickel cadmium or of the nickel metal hydride type. It relates in particular to a positive nickel electrode whose conductive support is constituted by a metal foam.
Until recently, electrochemical cells having an alkaline electrolyte included electrodes with a conductive support having active material deposited electrochemically or chemically into its pores by a plurality of successive precipitation operations. Unfortunately, that method of manufacture is lengthy and expensive. To satisfy the increasing needs of users, such electrodes can nowadays be made in a different manner.
An electrode having a non-sintered conductive support, also referred to as a pasted or plasticized electrode comprises a current collector which is coated in a paste containing the electrochemically active material and a binder, which are usually associated with one or more conductive materials. The conductive support can be two-dimensional or plane, as is the case for a solid or perforated foil, an expanded metal, a grid, or a cloth. The conductive support can also be three-dimensional such as a carbon or metal foam or felt which also acts as a structure for receiving the electrochemically active material, imparting mechanical strength to the electrode, and acting as a current collector. With three-dimensional current collector electrodes, the paste is introduced into the porous support which is then cut to the desired size. Cutting leaves strands of metal coming from the fibers constituting the support apparent at its edges. These subsequently run the risk of perforating the separator and giving rise to short circuits.
While an electrochemical cell is being assembled, the electrode needs to be electrically connected to the current outlet terminal. This connection is generally performed by means of a connection piece which is fixed firstly to the internal portion of the terminal and secondly to the conductive support of the electrode. The connection piece is preferably welded to the support so as to ensure that the electrical contact is reliable. When the electrode support is a three-dimensional support of large pore volume, its edge has neither the solidity nor the quantity of metal necessary to enable a connection piece to be welded directly thereto. Various solutions have been proposed for reinforcing the edge of the support prior to fixing the connection piece thereto.
The electrodes of opposite polarity are then assembled face to face on opposite sides of a separator. It is common practice, particularly in cells having spiral-wound electrodes, to offset electrodes of opposite polarities vertically relative to one another to a small extent so that the electrical connections between the electrodes of each polarity and the corresponding terminal are made at opposite ends of the roll.
In cells of the nickel metal hydride type, it has been found that the positive active material moves slowly or xe2x80x9ccreepsxe2x80x9d from the core of the electrode towards its surface and towards the edge of the electrode which does not carry the connection piece. After a certain length of time, the positive material projects from the edge of the separator and can come into contact with the negative electrode and/or with the negative connection piece. This contact leads under all circumstances to a leakage current being established which means that the cell has a high level of self-discharge, and it can also lead to short circuits. This phenomenon is particularly present with electrodes of large surface area used in high power cells that need to be capable of discharging quickly (complete discharge in less than 1 hour), such as those intended for cordless tools or for electric vehicles, for example.
Document JP-2-72564 describes an alkaline storage cell having spiral-wound electrodes such that the anode and the cathode project from respective opposite ends, one connection piece being welded to each projecting support. The cathode comprises a porous three-dimensional support into which the active material is introduced in the form of a paste. Internal short circuits can arise because of the positive active material swelling due to gas being given off during cycling. Such short circuits occur in the zone of the cathode which is adjacent to where the anode support projects. In order to remedy that problem, that document recommends introducing an adhesive into said zone that withstands the alkali and that replaces the active material to act as a barrier and prevent the active material from moving towards the anode connection piece. During manufacture of the electrode, the location reserved for the adhesive is marked. The support is filled with the paste containing the active material, then dried, compressed, and cut up. A molten resin is then cast into the reserved zone and hardens on cooling.
That solution presents various drawbacks. Firstly, the zone containing the adhesive must be free from any active material, which makes it necessary either to use a mask as described in that document, or else to remove active material that has penetrated therein during manufacture. Neither of those solutions is suitable for use on an industrial scale. Furthermore, the metal strands that are due to the cutting-out run the risk subsequently of giving rise to short circuits. The adhesive that is to be found inside the support does not enable such short circuits to be avoided.
An object of the present invention is thus to propose another solution for further reducing the risk of short circuits in an electrode of the non-sintered type and having a three-dimensional support.
A particular object of the invention is to propose an electrode of the non-sintered type with a three-dimensional support in which the phenomenon of creep of the active material over time is controlled so that no increase in self-discharge is observed throughout the lifetime of the cell.
The present invention provides an electrode comprising a porous three-dimensional conductive support containing an electrochemically active material, said support having at least a first edge connected to a connection piece and at least a second edge substantially parallel to said first edge, and means for preventing said active material disposed along said second edge from moving, wherein said means is selected from: a piece having a U-shaped fold placed stride said second edge; a surface covering; and treatment to modify the texture of said support.
With respect to an electrode, the term xe2x80x9cedgexe2x80x9d is used to designate that zone of the electrode which is constituted by its edge proper plus its surface margins contiguous therewith.
In a first embodiment of the invention, said means is a piece having a U-shaped fold placed astride said second edge.
Said piece can be selected from a tape and a portion of the separator. A piece in the form of a tape folded into a U-shape in the longitudinal direction can be fitted, or else a portion of the separator specially designed for this purpose can be used, which portion is folded over the edge of the electrode. Under such circumstances, the piece is made of the non-woven polyolefin material of the separator.
In a variant, said tape is made of an electrically insulating material. Said insulating material is preferably selected from a polymer, e.g. a non-woven material, or a cellulose compound such as paper.
By way of example, the selected polymer can be a polyolefin such as polyethylene or polypropylene, or it can be a polyamide.
In another variant, said tape is made of metal. The metal tape can be made of nickel, nickel-plated steel, or stainless steel, for example.
Said piece is preferably fixed on said electrode. It can be stuck on by means of adhesive or clips, and if it is a metal strip, it can be welded directly.
In a second embodiment of the invention, said means is a surface covering on said support. Said covering can be deposited by being sprayed or spread onto the surface of said support. The covering covers the surface of the support but does not penetrate into its pores.
Said covering is preferably selected from a polymer such as polytetrafluroethylene (PTFE), an elastomer such as styrene butadiene rubber (SBR), a varnish, a paint, or a paraffin.
In a third embodiment, said means constitutes treatment which modifies the texture of said support. In which case it consists in reducing the porosity of the support so as to create a barrier which retains the active material. Said treatment can be selected from compression and welding.
In a variant, said treatment further comprises folding said second edge. The support cleared of active material can be folded over the portion of the support containing active material, and then rolled to the thickness of the electrode.
Said three-dimensional conductive support is selected from a foam and a felt. Said conductive support is preferably a nickel foam.
Said electrochemically active material is a nickel hydroxide. The term xe2x80x9cnickel hydroxidexe2x80x9d means a hydroxide of nickel or a hydroxide containing mostly nickel hydroxide together with at least one syncrystallized hydroxide of some other element (Zn, Co, Ca, Cd, Mg, Mn, Al, . . . ).
The present invention also provides a secondary electrochemical cell including such an electrode which is a nickel positive electrode. Such cells are mainly cells having an alkaline electrolyte such as cells of the nickel metal hydride type, nickel cadmium type, nickel iron type, or nickel zinc type.
In a preferred embodiment of the invention, the secondary electrochemical cell is a cell of the nickel metal hydride type.
The present invention has the advantage of creating a mechanical barrier which prevents or contains displacement of the active material of the positive electrode. In addition, the barrier prevents the separator from being perforated by sharp strands projecting from the support.
The invention also has the advantage of enabling the active material contained in the support along the second edge to operate at least to some extent, with efficiency of about 40%, thereby conserving high capacity for the electrode of the invention.